1. Field of the Invention
The present invention relates to a magnetic recording medium comprising, as a magnetic recording layer, a ferromagnetic metal thin film which is suitable for high density magnetic recording, in particular, suitable for use in a digital video tape recorder or a high definition video tape recorder.
2. Description of the Related Art
In these years, as read/write equipment is miniaturized and made high quality in the magnetic recording field, magnetic recording media which enables high density recording have been vigorously developed. Recently, a continuous thin film type magnetic recording medium which is quite advantageous in short wavelength recording has been practically used in place of a particulate magnetic recording medium comprising a magnetic layer in which magnetic powder is dispersed in a binder resin.
With each of the above type magnetic recording media, in the writing and reading processes, a magnetic recording medium and a magnetic head are contacted and slid over each other. That is, a video tape or a floppy disc is continuously contacted to and slid over the magnetic head, while a magnetic disc is discontinuously contacted to and slid over the magnetic head. However, since a magnetic layer surface of the continuous thin film type magnetic recording medium is very smooth and has low hardness so that it is easily plastically deformed, a coefficient of friction of the magnetic layer against the magnetic head tends to increase. As a result, the magnetic layer tends to be abraded or damaged in the writing and reading processes.
Further, in a high humidity atmosphere, the magnetic layer surface is oxidized and corroded, so that the magnetic characteristics of the magnetic recording medium are deteriorated.
Therefore, the conventional thin film type magnetic recording medium has a protective film and a lubricant layer on the surface the magnetic layer to increase the running durability, namely to prevent damage of the medium due to contact sliding of the medium over the magnetic head, and to improved corrosion resistance, namely to suppress corrosion of the magnetic thin layer in a high humidity atmosphere.
Currently, as a non-magnetic substrate, a polyester film or a nickel-phosphorus plated aluminum disc is used. In some cases, the non-magnetic substrate has an undercoating layer with a filler or a texture consisting of a multiplicity of circumferential stripes to control surface roughness of the medium, if desired.
The magnetic layer consisting of the ferromagnetic thin film is formed from a ferromagnetic metal or alloy such as Co--Ni, Co--O, Co--Cr. Co--Cr--Ta, Co--Ni--Cr, Co--Pt, etc. by vacuum deposition, sputtering or the like.
As the protective film, there are used amorphous carbon (cf. Japanese Patent KOKAI Publication Nos. 142525/1986 and 208622/1986), diamond-like carbon (cf. Japanese Patent KOKAI Publication Nos. 219314/1987 and 210518/1986), an oxide such as SiO.sub.2 (cf. Japanese Patent KOKAI Publication No. 229743/1984), a plasma polymerized film comprising an organic compound or carbon (cf. Japanese Patent KOKAI Publication Nos. 171029/1984 and 89818/1985), and other materials disclosed in the references described below.
As the lubricant layer, a perfluoropolyether, a fluorine-containing carboxylic acid or a fluorine-containing phosphoric acid or other lubricant material is used.
Japanese Patent KOKAI Publication No. 29936/1985 (Document 1) discloses a magnetic recording medium having a diamond-like carbon film containing boron, silicon, etc. Japanese Patent KOKAI Publication No. 139871/1987 (Document 2) discloses a hydrogen-containing amorphous carbon film containing phosphorus and chromium. Japanese Patent KOKAI Publication No. 102018/1988 (Document 3) discloses a magnetic recording medium having a graphite carbon film containing 10% by weight or less of molybdenum sulfide and tungsten sulfide. Japanese Patent KOKAI Publication No. 217518/1988 (Document 4) discloses a magnetic recording medium having a graphite carbon film containing tin, indium, silicon, etc. Japanese Patent KOKAI Publication No. 237640/1985 (Document 5) discloses a magnetic recording medium having a plasma polymerized protective film comprising a hydrocarbon which contains a metal. Japanese Patent KOKOKU Publication No. 23926/1990 (Document 6) discloses a magnetic recording medium comprising a vapor phase polymerized layer comprising boron, carbon and hydrogen in which a content of boron decreases towards an interface between the polymerized layer and the ferromagnetic layer.
With the above structures, the running stability during contact sliding of the magnetic recording medium on the magnetic head and corrosion resistance in a high humidity atmosphere or a corrosive gas atmosphere may be increased, but are not necessarily increased satisfactorily.
For example, the plasma polymerized film which is formed as the protective film on the ferromagnetic metal layer as disclosed in Document 5 does not have good adhesion to the ferromagnetic metal layer. In addition, since the hardness of the plasma polymerized protective film is low, a sufficiently hard protective film is not provided. Accordingly, the magnetic recording medium is inferior in running durability.
When the various elements are added to the graphite carbon film as disclosed in Documents 3 and 4, the hardness of the protective film is insufficient. On the other hand, when the diamond-like carbon film is used as the protective film, the running durability is improved. However, since the diamond-like carbon is inactive, orientation and adhesion strength of the lubricant layer on the protective film are insufficient, so that water repellency of the magnetic recording medium is decreased. As a result, when the magnetic recording medium is kept in a high humidity atmosphere for a long time, the ferromagnetic metal layer is rusted. Further, the diamond-like carbon film and the ferromagnetic metal film are peeled off at the interface therebetween, so that output signal level is decreased. In addition, since orientation of the lubricant is insufficient, the lubricity of the magnetic recording medium is deteriorated. As a result, when the diamond-like carbon film is used as the protective film, the number of contact slides between the medium and the magnetic head increases, the coefficient of friction may increase. When the various elements are added to the hard diamond-like carbon film as disclosed in Documents 1 and 2, the hardness of the film is decreased, so that the running durability is deteriorated. Though the invention of Document 6 solves the above problems, the orientation and adhesion strength of the lubricant layer on the protective film are insufficient, and the running durability and corrosion resistance are still unstable.